In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for users (e.g. wireless terminal devices, or terminal devices for short) and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network-side (e.g. at transmission points or access nodes) and at the user-side might be required to reach a sufficient link budget.
At the transmission points hundreds of antennas elements are expected to be used for beamforming in order to counteract poor radio channel propagation properties.
In the document “Echo-MIMO: A Two-Way Channel Training Method for Matched Cooperative Beamforming”, by Lang P. Withers, Jr., in IEEE Transactions on Signal Processing, Vol. 56, No. 9, September 2008, there is a description of full loop precoding for a terminal device. The purpose of the precoding scheme is to attain channel state information (CSI) in the uplink from the terminal device when the terminal device is not calibrated between transmission and reception. The CSI can then be used at the terminal device for uplink precoding. The uplink estimation for the precoder described in this document is based on estimations of both round-trip channels and downlink channels. The uplink channel, which is used for determining the uplink precoder, is then estimated from the round-trip channels and downlink channels.
One issue with the above-described method is the amount of signalling needed in order for the terminal device to be able to determine the uplink precoder.
Hence, there is still a need for mechanisms enabling more efficient determination of the uplink precoder.